If you are still looking to add a few fish (be careful to not overstock) why not try a few sailfin mollies? They do excellent in brackish water and as a bonus would provide an occasional live food treat for the puffers. I keep my mollies in brackish (raise them as feeders) and they never get the diseases you see on mollies kept in freshwater. I have only sailfins, lyre-tailed, and Dalmatian mollies in brackish but some other may be tolerant to brackish. Just remember to drip acclimate when putting them into brackish.